Localized juvenile periodontitis (LJP) is the disease that occurs when bacteria accumulate on the junctional epithelium in the oral cavity. It was found in the middle of the last decade that Actinobacillus actinomycetemcomitans is the major causative agent of LJP and, more recently, as one of the microorganisms responsible for adult periodontitis. Periodontitis is the most prevalent chronic inflammatory diseases in humans, and it is major cause of tooth loss. It is evident that this dental pathogen grows actively and persists during the infectious process, which depends upon the ability of this microorganism to obtain growth-essential nutrients such as iron. Iron withholding by vertebrate hosts is an efficient mechanism against bacterial infections, and, thus bacteria must express efficient transport systems to acquire this essential nutrient to multiply during infection. There are indications that A. actinomycetemcomitans does not secrete siderophore compounds and it may acquire iron through periplasmic-binding protein-dependent transport systems. However, the genes and bacterial products involved in these processes and their role in virulence remain to be characterized. Therefore, the long-term objective of this application is the characterization of the mechanisms that A. actinomycetemcomitans uses to acquire iron and their participation in the pathogenesis of juvenile and adult periodontitis. In this proposal, the Principal Investigator addresses these goals through several approaches, combining methods used in classical and molecular bacterial genetics with molecular biology techniques designed to examine differential gene expression. The first specific aim involves a detailed genetic and molecular characterization of two potential iron periplasmic-transport systems, while the second specific aim focuses on the analysis of the expression of these systems in bacterial cells cultured in bacteriological media and in tissue culture flasks containing monolayers of human oral epithelial cells. The third specific aim proposes to determine the role of these potential iron transport systems in iron acquisition and the virulence of A. actinomycetemcomitans by creating and testing isogenic mutants. These proposed studies address an important and largely unexplored aspect of the pathogenesis caused by A. actinomycetemcomitans. Furthermore, these studies will lead to a better understanding of the nature of the interactions between the host and this pathogen during colonization and invasion of oral tissues.